1. Field of the Invention
The present invention relates to a beam-to-column connection, especially to a beam-to-column connection having high ductility.
2. Description of Prior Art
In the seismic resistant design of a building structure, the structural system and members should have enough strength to resist the forces generated during an earthquake. In order to prevent catastrophic collapse of the structure during a severe earthquake, stable and reliable energy dissipation capacity should be provided through proper design and detailing of the system, members and joints. For steel moment resisting frames under severe earthquake forces, it is usually assumed that the input energy is absorbed and dissipated primarily by the plastic hinge formed at the beam-to-column connection. The rotational capacities of beam-to-column connection greatly affect the energy dissipation capacity of moment resisting frames.
In structural analysis or design, emphasis is usually placed on either the individual structural member or the global behavior of the frame. For steel frames, the design and construction of their members is quite straightforward, since most of these members are pre-designed and fabricated structural shapes. Design guidelines have been well established for these structural shapes to fully develop their strength and ductility. However, the design and construction of the connections between these structural members is somewhat complicated. High strength bolts and/or welds are usually utilized in these connections. The structural behavior of these connections is not only affected by the geometric changes around connections but also affected by drilling of bolt holes and welding. Improper design and/or construction of these connections may lead to the collapse failure of the whole structure.
Research on the characteristics of connections is usually conducted through large scale structural testing to account for the effects of bolting and welding. In general engineering practice, the steel beam, such as an H-beam having a pair of parallel flanges connected by a web, is usually connected to the column by bolting the beam web to the shear tab on the column plate and then welding the beam flange with the column plate to form a moment connection. This is widely used and has almost become standard practice. However, from large scale experimental studies, lack of deformation capacity of this type of connection was reported by S. J. Chen (The Chinese Journal of Mechanics, Vol. 6 No. 2, 1990), S. J. Chen (Journal of the Chinese Institute of Engineers, Vol. 16, No. 3, pp. 381-394, 1993).
The ductility (i.e. the deformation capacity or the energy dissipation capacity) of a connection can be represented by plastic rotational angle .THETA.p. In order to define the plastic rotational angle .THETA.p, a cantilever beam is chosen for illustration as shown in FIG. 1. A cantilever beam supporting a concentrated load P at its free end is constructed of a elastic-plastic material. It is customary that a downward load is a positive load and an upward load is a negative load. In FIG. 1, the load is a positive load. Assuming the cantilever beam has a total deflection .delta. and elastic deflection .delta.e, then the plastic deflection of the cantilever beam .delta.p is equal to .delta.-.delta.e. Also, assuming the length of the cantilever beam is L, the plastic rotational angle .THETA.p is defined as following: EQU .THETA.p=Min[Abs(.THETA.p')]
wherein .THETA.p'=.delta.p/L. From the abovementioned definition, it is understood that the larger plastic rotational angle a beam has, the larger ductility a connection of the beam has. FIG. 2 shows statistical data for 37 large size steel beam to box-column connections subjected to cyclic load tests in recent years in Taiwan. Out of these 37 specimens, about 20% (8 specimens) were brittle. The average plastic rotational angle of these specimens was only 0.92% radian which was not adequate.
Recent work done by Engelhardt (J. of Structural Division, ASCE, vol. 119, No. 12, 1993) also found lack of reliable deformation capacity of the beam to H-column moment connection. The reliability of field welding has long been questioned. Some people prefer to weld a short beam to the column in the factory and use all-bolt beam splices in the field. However, this would increase the construction cost substantially. Cover plates are also selected to increase the flexural strength at beam-to-column connections and to move the critical section away from the field welding. But the use of cover plates usually increases the amount of field welds and in-turn increases the possibility of welding defects.
During the Northridge Earthquake in California in 1994, a number of steel buildings suffered from fractures in welded beam-to-column connections of moment resisting frames. The performance of these steel frames during the earthquake raised questions about the quality assurance of field welding. It seems that the current welding design is susceptible to failure in earthquakes.